“Color gamut” is a term used to describe the range of colors available in a particular system. Different types of display systems tend to each have their own specific color gamut, depending upon their technological limits. Oftentimes, in visual display systems (such as digital micromirror devices, plasma displays, or LCDs, used as video displays, computer screens, or projectors, for example) the input or source color gamut differs from the output or display color gamut. This may be the case if, for example, the input signal was recorded based on a standard Rec. 709-based monitor (source device), while the actual display device (target device) used to reproduce the visual images associated with the input signal is a digital micromirror device (DMD)-based projection display system with a potentially different color gamut. As a result, some of the recorded colors from the input signal may not be reproducible within the target color gamut. Likewise, the target color gamut may allow access to a range of colors not available with the source gamut, providing the ability to more faithfully reproduce the original image.
By way of example, the target display device may have the ability to reproduce a wider array of colors than the source device. In such an instance, the target device could simply reproduce the images associated with the input signal precisely as the source might, but this would mean that the target device would not be using its full capabilities; much of the target's expanded color range would be wasted. In such an instance, the target would be capable of reproducing the visual image much more fully if it were allowed to utilize its expanded color gamut. By effectively mapping the more limited source gamut onto the target color gamut, more accurate color reproductions may be possible. If on the other hand, the input signal from the source device has a larger color gamut than does the target device, then the target display device would be incapable of recreating the visual image precisely in accordance with the input signal of the source device. In such instances, it may be useful to be able to map the source device's color gamut onto the target device's color gamut, in a way that provides as accurate a reproduction of the visual image as is possible given the constraints of the device.
With all of the different display choices currently available (each having a different, unique color gamut), the ability to effectively map from one color gamut to another is increasingly important. Only by using an effective color mapping technique can visual images be faithfully reproduced in a way that provides a realistic and pleasing image. Furthermore, an effective color mapping scheme may allow for the full color gamut of a display device to be utilized in reproducing images. This allows display technology to be maximized, taking full advantage of the available color gamut.
Conventional mapping systems consider color gamuts within a non-perceptual x-y chromaticity space, such as the exemplary space illustrated by the x-y chromaticity diagram shown in FIG. 1. This type of color space ignores luminance in its computations explicitly (although it may be included implicitly), mapping the color gamut in terms of two color spectra. The color gamut of human perception is shown as an horse-shoe-shaped figure on the x-y chromaticity diagram. The color gamut of any display device that utilizes three primary colors to reproduce a range of colors (in the manner of a CRT for example) may be represented as a triangle within the horse-shoe-shaped figure. The actual colors that any such display device can reproduce are less than the entire scope of perceptible colors, and the color gamut of the display device may be illustrated as the space within the triangle.
Mapping from one color gamut to another within the x-y chromaticity space is limited by its very nature to a linear or piecewise-linear projection due to the linearity of the XYZ color space. Linear or piecewise-linear mapping between color gamuts may be restrictive, preventing effective color mapping that achieves calorimetric accuracy. By way of example, linear mapping tends to compress “core” colors (which are the achromatic or memory colors) distorting their rendition. FIG. 2 provides an illustrative example of the type of mapping conventionally performed within the x-y chromaticity space. The three end points of the input triangle (color gamut) would be linearly mapped onto the end points of the output color gamut, with any other points within the triangular color gamut linearly following that mapping. In other words, each point within the input color gamut would be stretched in a linear (or piece-wise linear) fashion, thereby projecting onto the nearest approximation within the output color gamut. While this type of linear color mapping may be adequate if color gamuts within the x-y chromaticity space are sufficiently similar, linear mapping is often not particularly effective, possibly distorting colors so that the image colors are not reproduced particularly faithfully. The goal is to preserve calorimetric accuracy, and linear mapping may not be able to achieve this result given its constraints.
In addition, such a conventional linear mapping system may not effectively preserve memory colors. Memory colors are specific colors that are particularly important to a viewer's perception of reality. Any distortion of a memory color perceived by a viewer tends to drastically reduce the believability of the reproduced image. Examples of memory colors might be the colors associated with the blue of a clear sky, the green of grass, or assorted flesh tones of people's skin. These are real-world colors, and human beings have an innate feeling for what these colors should look like. Whenever a display device does not accurately reproduce such memory colors, the image tends to be viewed as unrealistic. Conventional color mapping systems may not effectively maintain or reproduce such memory colors, resulting in a less effective image.